/*
https://leetcode.cn/problems/binary-tree-preorder-traversal/submissions/584024534/
144. 二叉树的前序遍历
*/
/**
 * Definition for a binary tree node.
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode() : val(0), left(nullptr), right(nullptr) {}
 *     TreeNode(int x) : val(x), left(nullptr), right(nullptr) {}
 *     TreeNode(int x, TreeNode *left, TreeNode *right) : val(x), left(left), right(right) {}
 * };
 */
class Solution {
public:
    
    vector<int> preorderTraversal(TreeNode* root) {
        vector<int>res;
        cal(root,res);
        return res;
    }
    void cal(TreeNode* root,vector<int>& res){
        if(root==NULL) return ;
        res.push_back(root->val);
        if(root->left!=NULL) cal(root->left,res);
        if(root->right!=NULL) cal(root->right,res);
    }

};